The present invention relates generally to the field of wireless communications and specifically to an improved automatic frequency control circuit and method.
Automatic frequency control (AFC) is a fundamental process for radio communication devices. AFC typically includes one or more phase-locked loop (PLL) circuit(s), which generate a high frequency periodic signal from a low frequency reference signal. AFC is used to perform a wide variety of tasks in wireless communication system components, such as frequency synthesis, AM and FM detection, frequency multiplication, tone decoding, pulse synchronization of signals from noisy sources, and the like, with high frequency accuracy.
One conventional, representative application of AFC, described herein to explicate the present invention, is in conjunction with the downconversion of a received Radio Frequency (RF) signal, such as that produced by the transmitter at a base station, to the baseband frequency in a receiver, such as a wireless communication mobile terminal, for demodulation of the RF signal. The downconversion is accomplished by multiplying (mixing) the RF signal with a high frequency signal (often referred to in the art as a Local Oscillator (LO) frequency signal) to produce sum and difference components. The resulting analog I and Q signals of the difference component (representing the In-phase and Quadrature-phase components, respectively) may be digitized and processed by a baseband processor, such as an appropriately programmed Digital Signal Processor (DSP), to extract information symbols from the signal, as is well known in the art. The LO signal is typically the output of a phase locked loop, which typically uses a high quality crystal (XTAL) oscillator, the frequency of which is multiplied by a programmable factor by the phase locked loop (PLL) circuit.
The residual frequency offset between the actual RF signal and the desired RF frequency (due to the XTAL oscillator frequency error) is an error that should be eliminated or minimized to within an allowable tolerance. This frequency error can be determined in the baseband processor from the speed of rotation of the (I,Q) constellation. This frequency error (in digital representation) is typically converted to an analog voltage by a Digital-to-Analog Converter (DAC) and applied to a frequency control circuit of the XTAL oscillator generating the PLL's reference frequency. In particular, the output voltage of the DAC may be applied to a Variable Capacitance diode (Varicap) through a series of decoupling resistors and capacitors to change the capacitance in the overall resonant circuit, and thereby change the frequency of oscillation slightly. This slight change in the XTAL oscillator circuit output frequency is multiplied by the PLL, resulting in a corresponding change in the local oscillator frequency. The difference between this altered LO frequency and the RF signal is again detected at the baseband processor, and the XTAL oscillator reference frequency again adjusted, in a closed-loop feedback manner. This frequency control loop is commonly referred to as Automatic Frequency Control (AFC).
The voltage level fed back to the XTAL oscillator tuning circuit is not, in general, a direct function of the detected frequency error. Each element in the XTAL oscillator tuning circuit introduces errors due to component tolerance, aging, reflow soldering effects, and the like; furthermore, many of these errors change non-linearly with temperature. Typically, the XTAL oscillator tuning circuit is extensively characterized at the factory, and control voltages to effect various frequency shifts are stored in look-up tables in the baseband processor. Additionally, the baseband processor may dynamically alter or add to the look-up table entries to reflect XTAL oscillator tuning circuit control voltages and associated temperatures and/or other operating characteristics, to build up a more robust error frequency control model over time.
The XTAL oscillator tuning circuit components and DAC add cost to the mobile terminal, consume printed circuit board space, and decrease reliability by providing additional failure points. The extensive characterization of the XTAL oscillator tuning circuit to create initial look-up table entries is time consuming and costly. Finally, the dynamic adaptation of the look-up table entries consumes limited processing resources and adds complexity to the radio control software.